In cognitive radio systems, cognitive radio equipment uses an incumbent user's frequency band (or frequency channel) when the incumbent user does not occupy the frequency band. Examples of cognitive radio equipment are base stations and customer premises equipments (CPEs), which are included in cognitive radio systems. Incumbent users may be referred to as primary users. Cognitive radio systems may be referred to as secondary user systems.
In cognitive radio systems, spectrum sensing technology examines the spectrum of a frequency band in order to allow cognitive radio equipment to use an incumbent user's frequency channel and check if the incumbent user appears in the frequency channel when the cognitive radio equipment uses the frequency channel.
Cognitive radio systems basically must guarantee the vested rights of incumbent users to use their own frequency channels as much as they can. To this end, another spectrum sensing technology for more accurately detecting whether an incumbent user's frequency channel is preoccupied by cognitive radio equipment, i.e., whether an incumbent user appears in the frequency channel, is needed.